Several methods for modifying or coating the surface of sheet material were known in the art, for example, plasma treatment by glow discharge in vacuum, plasma polymerization, and plasma chemical vapor deposition. Typical of such known methods for the continuous surface treatment of sheet material are a batch system in which a length of sheet material is continuously unraveled from its roll in a vacuum chamber where the sheet surface is continuously treated, and a differential evacuation system in which the chamber is gradually evacuated from atmospheric pressure to a vacuum as disclosed in Japanese Patent Publication No. 65093/1988. These methods, either batchwise or continuous, use a large size of equipment and require a pump of an increased capacity for evacuating the large chamber.
It is also known to apply electric discharge treatment to the surface of sheet material in a gas mixture under atmospheric pressure as disclosed in Japanese Patent Application Kokai No. 235339/1987, and Japanese Patent Application Nos. 138630/1988, 166599/1988, and 202977/1988. All these methods are batchwise. Although treatment under atmospheric pressure is advantageous, shielding from the ambient atmosphere by a chamber is necessary. For continuous treatment, a roll of sheet material should be placed in a chamber, for example, a large size closed tank where treatment is carried out. Therefore, the size and attachments required for the equipment are basically the same as in the aforementioned vacuum treatment.
There is a demand for method and apparatus capable of continuously modifying or coating the surface of a sheet material in a uniform fashion without a need for a large size closed tank or a pump of high capacity.